1. Field of the Invention
The present invention relates to a controller for a motor that has two rotors each having a permanent magnet to produce a field and can change the phase difference between the rotors.
2. Description of the Related Art
As a permanent magnet type motor, there has been known a double rotor type motor that has two rotors that are coaxially disposed and each have a permanent magnet for producing a field (for example, see Japanese Patent Laid-Open No. 2002-204541 (referred to as Patent Document 1 hereinafter)). In the motor of this type, the two rotors can relatively rotate with respect to each other about the axis thereof, and the phase difference between the rotors can be changed by the relative rotation. By changing the phase difference between the rotors, the strength of a composite field (the magnitude of magnetic fluxes) of the fields produced by the permanent magnets of the rotors can be changed.
The motor disclosed in the Patent Document 1 is mechanically configured so that the phase difference between the rotors changes according to the rotational velocity of the motor. That is, the rotors are connected to each other via a member that is displaced in the radial direction of the motor by the action of a centrifugal force. One of the rotors is capable of integrally rotating with the output shaft of the motor for outputting the torque produced by the motor to the outside. When the member described above is displaced, the other rotor relatively rotates with respect to the rotor capable of rotating integrally with the output shaft, and thus the phase difference between the rotors changes. In this case, the permanent magnets of the rotors are disposed in such a manner that, when the motor is halted, the directions of the magnetic poles (the directions of the magnetic fluxes) of the permanent magnets of the rotors are the same, and the strength of the composite field of the permanent magnets is maximized. As the rotational velocity of the motor increases, the phase difference between the rotors changes by the action of the centrifugal force, and the strength of the composite field of the permanent magnets of the rotors decreases.
The motor disclosed in the Patent Document 1 simply mechanically changes the phase difference between the rotors according to the rotational velocity of the output shaft. Therefore, for example, in an operational state where the rotational velocity of the output shaft is high, the phase difference between the rotors is such a phase difference that the fields of the rotors destructively interfere with each other. If such an operational state continues for a long time, demagnetization of the permanent magnets of the rotors is likely to occur. In addition, demagnetization of the permanent magnets of the rotors is likely to occur when the temperature thereof is excessively high. If such demagnetization occurs, the performance of the motor can be degraded, and it can be difficult to make the motor operate in a desired way.